Motor vehicles with a thermal engine conventionally comprise an on-board electrical network comprising a battery, generally a 12 V battery, which is designed to supply the various items of equipment with electrical energy, in particular a starter, which is essential in order to ensure starting of the thermal engine. After the starting, an alternator which is coupled to the thermal engine ensures that the battery is charged.
Nowadays, the development of power electronics has made it possible to supply and control a single reversible polyphase rotary electrical machine which advantageously replaces the starter and the alternator.
In a first stage, this machine, which is known as an alternator-starter, essentially had the purpose of fulfilling the functions previously dedicated to the alternator and the starter, and, in addition, of recuperating energy from the braking, or providing the thermal engine with additional power and torque.
For the purpose of increasing the power and improving the performance of the alternator-starter by increasing its operating voltage, whilst maintaining the possibility of using other standard equipment designed for a 12 V to 14 V supply, in particular the lead batteries, a so-called “14+X” or “micro-hybrid” architecture has been developed.
This architecture thus consists of an electrical power network which connects the alternator-starter to an electrical energy storage element which functions at a voltage higher than 14 V, and can be as much as 48 V, and of an electric service network which connects all the other equipment. The adaptation of the voltage levels between the two networks is ensured by a reversible direct-direct converter.
In a second stage, ecological considerations led to design of alternator-starters with power of approximately 8 to 10 kW, which is sufficient to drive the vehicle at low speed, for example in an urban environment.
Power levels of this type have been able to be obtained, whilst continuing to have compact electrical machines, only by increasing the voltage of the electrical power network to a voltage of approximately 60 V, which is far higher than the nominal voltage of conventional lead batteries.
Furthermore, power networks with voltages of up to 120 V can be implemented in an architecture which allows the vehicle to be driven at full speed by the electric motor (so-called full-hybrid architecture, as opposed to the previous so-called mild-hybrid architecture).
In order to carry out the functions specific to the aforementioned hybrid vehicles, a large amount of power is supplied essentially by the storage element of the power network.
During a recuperative braking phase, the energy restored must be absorbed rapidly by the high-voltage battery, and, conversely, during torque assistance phases, the high-voltage battery must be able to supply a large amount of power. A storage element of this type must therefore have very low internal resistance, in order to avoid voltage losses during the discharging phases, and excess voltages during the charging phases.
At the same time, it must have an energy level which is sufficient to be able to supply energy in a travelling phase purely in electric mode (known as ZEV, i.e. Zero Emission Vehicle), and it will be appreciated that the quantity of energy available over a long period is of primordial importance.
However, in the present state of the art, there are no electrical energy storage devices which have both strong specific power and substantial specific energy. Use is habitually made in a non-optimum manner of lithium-ion batteries which are subject to severe constraints which limit their reliability and service life.
In the article “Improvement of Drive Range, Acceleration and Deceleration Performance in an Electric Vehicle Propulsion System” presented by X. Yan et al during the 99 PESC conference (“30th annual IEEE Powers Electronics Specialists Conference”, 1999, Vol. 2, pages 638-643), zinc-bromine batteries are described which are optimised in terms of their specific energy, their service life, and their low-cost, associated with ultra-capacitors which provide the power peaks.
A two-way direct-direct converter controls the charging/discharging of the two types of storage devices, according to the operative states of the vehicle (acceleration, overtaking, recuperative braking, etc.).
This converter comprises a power semiconductor half-bridge and an induction coil which are connected respectively to the ZnBr batteries and to the ultra-capacitors, such as to constitute a step-up/step-down assembly.
Control of the semiconductors of the half-bridge is simple, but in this architecture, these semiconductors are subjected to the voltages which are present at the terminals of the storage devices and on the power network, and must therefore switch large amounts of power. The cost of these semiconductors can then be high.